1. Field of the Invention
The present invention relates to chemical heat pumps utilizing the phenomenon of absorption and release of the heat of reaction of a working fluid substance, in order to raise or lower the temperature of a thermal medium so as to effect air conditioning, refrigeration, and the like. The heat of reaction includes the latent heat of phase transition and heat of absorption or evolvement, as well as the heat of usual chemical reactions.
2. Description of the Prior Art
Industrial waste heat consists largely of low temperature waste heat at a temperature of roughly 30 to 50 degrees centigrade. Further, underground water represents a vast heat source with a temperature of about 15 to 20 degrees centigrade. Thus, if it is possible to raise or lower the temperature of a thermal medium to a practical level using only these low quality heat sources, the resulting advantage is very great.
One type of heat pump apparatus which has been proposed for utilizing such heat sources is the steam compression-type heat pump. This type of heat pump draws up heat by shifting the equilibrium of the working fluid between the gas and the liquid phase by means of a compressor which is driven, for example, by an electric motor. The steam compression-type heat pump is capable of obtaining high quality heat in a temperature range of about 50 to 60 degrees centigrade by raising the temperature of low quality heat having a temperature of about 15 to 20 degrees centigrade.
However, the coefficient of performance of these steam compression-type heat pumps is rather low, being only about 3 in the above-described case.
Thus, there is much need for chemical heat pumps which utilize only the temperature difference between high and low temperature heat sources (e.g., between waste heat or underground water and open air) and do not need to be supplied with mechanical work in order to operate.
Referring now to FIGS. 1 through 3, the principles of operation of conventional chemical heat pumps will be explained.
FIG. 1 is a graph illustrating the relationships between the reciprocal (1/T) of the absolute temperature T and the logarithm (log P) of the equilibrium pressure P for absorption and evolvement reactions in which a working fluid (such as ammonia or water) is absorbed into and evolved from two distinct absorbents X and Y. Line X represents the relationship between the reciprocal (1/T) of the absolute temperature T and the logarithm (log P) of the equilibrium pressure P of the working fluid (in pure chemical composition) when there is equilibrium between the liquid and the gas phases. As shown in the figure, the logarithm (log P) of the equilibrium pressure P varies substantially linearly with respect to the reciprocal (1/T) of the absolute temperature T. The equilibrium pressure at a given temperature, however, takes different values according to the kinds and concentrations of the absorbents used.
FIGS. 2 and 3 are schematic views of a heat pump for raising the temperature of a thermal medium utilizing absorbents X and Y which have differing equilibrium pressures. Absorbent X may be replaced with the liquid phase of the working fluid. Thus, the following description, which is made with respect to the case of two distinct absorbents, also applies to the case where phase equilibrium between the gas and the liquid phases of the working fluid is utilized instead of absorption-evolvement equilibrium of the working fluid into and from absorbent X.
The pair of reaction vessels containing the absorbents X and Y are connected by means of a gas pipe so that the gaseous working fluid G is capable of being transported between the two vessels. The absorbents X and Y have the equilibrium pressure-temperature characteristics shown in FIG. 1 by lines X and Y, respectively. FIGS. 2 and 3 show the temperature raising process and the regeneration process, respectively. Utilizing the first heat source 1, such as industrial waste heat at a temperature T1, and the second heat source 2, such as open air at a temperature T2 which is lower than T1, the heat pump raises the temperature of the thermal medium 3 to a target level T3 which is higher than the temperature T1 of the first heat source 1.
In the temperature raising process shown in FIG. 2, the absorbents X and Y are placed in thermal contact with the first heat source 1 and the medium 3, respectively. Thus, the gaseous working fluid G evolving from absorbent X having a high equilibrium pressure moves in the gas pipe in the direction shown by the arrow in FIG. 2, and is absorbed by absorbent Y. As shown in FIG. 1, this process proceeds at equilibrium pressures substantially equal to P1, the driving force acting on the gaseous working fluid G being derived from the slightly higher equilibrium pressure of absorbent X with respect to that of absorbent Y. In the process, the working fluid G absorbs the heat of gas evolvement .DELTA.H1 from the first heat source 1 and supplies the heat of gas absorption .DELTA.H2 to the medium 3, so that the temperature of the medium 3 is raised to the target temperature T3 which is higher than the temperature T1 of the first heat source 1.
On the other hand, in the regeneration process shown in FIG. 3, absorbent Y is in thermal contact with the first heat source 1, while absorbent X is in thermal contact with the second heat source 2 which is the coolant heat source. Thus, the temperatures of the absorbents X and Y are made substantially equal to the temperatures T2 and T1 of the second and the first heat sources 2 and 1, respectively. During this regeneration process, the equilibrium pressure of the working fluid G is substantially equal to the pressure P2 of both absorbents X and Y as shown in FIG. 1. The equilibrium pressure of absorbent Y, however, is slightly higher than that of absorbent X, and this pressure difference drives the working fluid G evolved from absorbent Y in the direction shown by the arrow in FIG. 3 in the gas pipe so that it is absorbed by absorbent X. In the process, absorbent Y gives up the heat of evolvement .DELTA.H3 to the working fluid G, and the working fluid G releases the heat of absorption .DELTA.H4 to absorbent X.
Thus, in principle, chemical heat pumps are capable of raising the temperature of a thermal medium utilizing only the temperature difference between the two heat sources without any need of mechanical work. Because chemical heat pumps need scarcely any external mechanical power, they are potentially capable of greatly reducing energy consumption if they can be applied to air-conditioning of dwellings, greenhouse heating, and the like.
Although the above description has been limited to the case of a temperature-raising apparatus, chemical heat pumps can also be used to lower the temperature of a thermal medium, i.e., for cooling or refrigerating purposes, in which the directions of the arrows in FIGS. 1 through 3 are reversed.
However, conventional chemical heat pumps have a grave disadvantage that they are not capable of obtaining high quality heat when a low quality heat source with a temperature of about 20 to 50 degrees centigrade is used. For example, when waste hot water at 30 degrees centigrade is used as the first heat source 1 and the open air at 10 degrees centigrade is used as the second heat source 2, the thermal medium 3, such as hot water reaches a temperature of only about 40 degrees centigrade, making the hot water of little value. Even if waste steam at 40 degrees centigrade is used as the higher temperature heat source 1, the hot water obtained reaches only 55 degrees centigrade. Thus, the change in temperature which conventional heat pumps can produce when using low quality heat sources is extremely small, which severely limits their utility.
The reason why the change in temperature which conventional heat pumps can produce is small is as follows. (Although the case where a heat pump is used to raise the temperature of a thermal medium is explained, the explanation also applies to the case where the temperature of the medium is lowered.)
The temperature rise .DELTA.T of the thermal medium which is produced by the heat pump of FIGS. 2 and 3 is equal to T3-T1. It is now assumed that lines X and Y representing the relationships between the reciprocal (1/T) of the absolute temperature T and the logarithm (log P) of the equilibrium pressure P of the working fluid G when it is in equilibrium with absorbents X and Y, respectively, are parallel to each other, as shown in FIG. 1. This assumption is substantially justified in the case of conventional chemical heat pumps as explained later. Under this assumption, the following equation holds: EQU (1/T3)-(1/T1)=(1/T1)-(1/T2).
Thus, EQU .DELTA.T=T3-T1=(T3/T2) (T1-T2).congruent.T1-T2.
Thus, the rise in temperature .DELTA.T is substantially limited by the difference in temperature T1-T2 between the first and the second heat sources 1 and 2 cannot exceed it. The loss of heat in actual heat pumps further reduces the temperature rise .DELTA.T which the pump can produce in the thermal medium.
In the above explanation, the lines representing the relationships between the reciprocal (1/T) of the absolute temperature T and the logarithm (log P) of the equilibrium pressure P of the working fluid when there is equilibrium with two kinds of absorbents X and Y were assumed to be parallel to each other. Justification of this assumption is as follows.
The slope of the line representing the relationship between the reciprocal (1/T) of the absolute temperature T and the logarithm (log P) of the equilibrium pressure P at an equilibrium in chemical reactions, including gas absorption and evolvement by absorbents or phase transition between liquid and gas phases, is solely dependent upon the heat of reaction per mole .DELTA.H/mol, or more specifically is equal to (.DELTA.H/mol)/R wherein R is the gas constant.
The magnitudes of the heat of reaction per mole of working fluids such as ammonia which are used in conventional chemical heat pumps, however, are substantially the same for different kinds of absorbents, and for the gas absorption reaction and the phase transition reaction. The magnitudes of the heat of reaction are particularly close to each other in the case of the pair of absorbents used in conventional heat pumps, because the temperatures at which a practical pair of absorbents attain equilibrium under 1 atmosphere should be close to each other. Thus, in the case of a practical pair of absorbents, the lines representing the relationship between the reciprocal (1/T) of the absolute temperature T and the logarithm (log P) of the equilibrium pressure P are substantially parallel to each other.
Therefore, the change in temperature which conventional chemical heat pumps are capable of producing is severely limited even in principle. If the heat loss in actual pumps is taken into consideration, chemical heat pumps utilizing low quality heat sources are unpractical.